13C-19F coupling constants

Few other studies are available on one-bond 13C-H or 13C-19F coupling constants as a function of solvent. Rahkamaa and Jokisaar 27> reported the solvent and temperature dependence of XJXZ in ethyl formate in three solvents CC14,... 

Table 19a 13C-19F Coupling Constants" in Ethyl 6-Cyano-4,5,7-trifluoro-2-methylbenzo[6]furan-3-carboxylate...

Table 4.20. (6) 13C — 19F Coupling Constants (in Hz) of Methyl-2,2-difluoronorbornanes and exo-2-Fluoronorbornane [229].

As can be seen from Table 5.9, the 13C — 19F coupling constants decrease drastically with increasing distance of the carbon atoms from the CF group in the molecule. With the exception of C-19, fluorine substitution causes the y-carbon signals to shift upheld (cf. resonances of C-l, C-5, C-7, C-12 and C-14 of the compounds in Table 5.9). 

K. Bock and C. M. Pedersen, A study of 13C-19F coupling constants in glycosyl fluorides, Acta Chem. Scand. B, 29 (1975) 682-686. 

Pairwise interaction parameters can perhaps be interpreted in chemical bonding terms (76, 80,123, 182). Pairwise terms involving fluorine are often anomalous and this may be related to some surviving multiple bonding in the boron-fluorine bonds in the adducts (20). Pairwise interaction parameters of the Me3N mixed trihalide adduct system are reported to correlate consistently for 41, 13C, 19F, and nB chemical shifts and for nB—19F coupling constants (123). It is perhaps surprising that even the proton shifts can be correlated in this way. 

The complete set of NMR data for one series of o-, m-, and p-disubstituted fluorobenzene compounds, that of the nitrofluoroben-zenes, will serve to further exemplify the interplay of 19F, 13C, and H chemical shifts and coupling constants that provide unique insight into the structures of disubstituted fluorobenzenes. These data are given in Tables 3.3-3.5. 

The influences of SF5 groups upon H and 13C NMR spectra are also quite characteristic, and information about proton and carbon chemical shifts and the respective coupling constants will be included along with the 19F data where they are available. The proton and carbon NMR spectra for (pentafluorosulfanylmethylene)cyclohexane are provided in Figs. 7.3 and 7.4 as typical examples. 

The signs of the geminal, 19F-13C coupling-constants both for glyco-pyranosyl and glycofuranosyl fluorides were found to be positive,278 whereas, in another investigation, the spectra were considered to be too complex for determination of the signs, but, by comparison with noncarbohydrate systems, they were assumed to be positive.279... 

Fluorine NMR data can be collected readily on most spectrometers, requiring only minor adjustments to instrumentation used to run proton samples The fluonne-19 nucleus is easily detected (relative abundance, 100%, spin, 1/2) and generates a wealth of spectral information to assist in structure elucidation To take full advantage of all the spectral evidence available, lH, 13C, and 19F chemical shifts and coupling constants should be acquired and correlated... 

Pople et al 27) developed the INDO method specifically to account for hyperfine coupling constants in terms of spin densities. Pople, Beveridge and Dobosh have calculated the spin densities of a large number of compounds and found a good agreement with the experimental isotropic hyperfine coupling constants of 1H, 13C, 14N, 170, and 19F (Tables 32 to 35). 

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